Diet and lifespan A calorie-restricted diet does not increase lifespan, but does have some health benefits, according to a long-term study of rhesus monkeys.

The research contradicts similar studies that claim that cutting calories does boost longevity.

Dr Julie Mattison of the US National Institute on Ageing (NIA), and colleagues studied the effects of calorie restriction for the past 23 years on monkeys in two age groups - a young onset group (animals that were 1 - 14 years old when enrolled in the study) and an old-onset group (aged 16 - 23 when enrolled).

Their study, published today in the journal Nature, found that restricting the monkeys' caloric intake at older ages did not increase survival, but it did improve the health and function of their metabolism.

In the old-onset group both male and female monkeys on a calorie restricted diet weighed less than their control counterparts, although the diet effect was greater in the males. Triglycerides were significantly lower in the CR monkeys and cholesterol remained significantly lower in the CR males.

Young monkeys on a calorie restriction diet showed a trend towards a delay in age-associated disease onset, but again, no increase in lifespan, they say. Significantly the incidence of cancer was markedly reduced in the young-onset CR monkeys and the glucoregulatory function was also improved.

Contradictory findings

The findings contradict findings from a parallel study using rhesus monkeys by the Wisconsin National Primate Research Centre (WNPRC) that shows a clear trend toward longer life from calorie restriction.

Mattison and colleagues suggest study design and diet composition is a major difference between the two studies and could account for the disparate findings.

This view is supported in an accompanying commentary by Dr Steven N. Austad, of the University of Texas Health Science Center, who writes that while the diets are broadly similar in their overall content of carbohydrates, proteins and fats, they differ in the specific types of such nutrients.

"For instance, sucrose made up 28.5 per cent of the WNPRC diet, but only 3.9 per cent of the NIA diet. Possibly related to this difference, more than 40 per cent of the WNPRC control animals and only 12.5 per cent of the NIA controls developed diabetes," writes Austad.

Another major difference is that the NIA controls were given a regulated amount of food to prevent obesity, whereas the WNPRC controls could eat as much as they pleased.

Austad says the NIA control animals weighed less and were considerably longer-lived than the WNPRC controls.

"One interpretation of this observation is that the NIA controls were partially restricted," he says, adding this would account for the lack of effect on lifespan in the NIA study.

A different take

University of Sydney's Professor Stephen Simpson and Massey University's Professor David Rubenheimer suggest there is another way of looking at the issue altogether.

In their book The Nature of Nutrition they say it's not that calorie-controlled monkeys live longer, but rather monkeys with unrestricted access to food die sooner.

"When considering ageing and lifespan, most diet restriction experiments use two treatment regimes: food restricted and unrestricted," they write.

"If animals live longer when food restricted, the usual interpretation is that there are benefits to eating less."

"[F]ood-restricted animals don't live longer because of the benefits of restriction, but rather, the other group dies sooner because of the damage caused by unrestricted access to food," they write.

Simpson and Rubenheimer also point out that dietary composition requirements change during the ageing process, yet in these experiments the composition of the diet is fixed.

"It follows [then] the closer a diet matches the changing needs of an individual the less beneficial will be the effects of diet restriction. In fact, a recent study in rodents saw no change in lifespan in response to food restriction when animals were confined to a food that was 'optimised for health and longevity benefits."